Cholesterol is used for the synthesis of bile acids in the liver, the manufacture and repair of cell membranes, and the synthesis of steroid hormones. There are both exogenous and endogenous sources of cholesterol. The average American consumes about 450 mg of cholesterol each day and produces an additional 500 to 1,000 mg in the liver and other tissues. Another source is the 500 to 1,000 mg of biliary cholesterol that is secreted into the intestine daily; about 50 percent is reabsorbed (enterohepatic circulation). Excess accumulation of cholesterol in the arterial walls can result in atherosclerosis, which is characterized by plaque formation. The plaques inhibit blood flow, promote clot formation and can ultimately cause heart attacks, stroke and claudication. Development of therapeutic agents for the treatment of atherosclerosis and other diseases associated with cholesterol metabolism has been focused on achieving a more complete understanding of the biochemical pathways involved. Most recently, liver X receptors (LXRs) were identified as key components in cholesterol homeostasis.
The LXRs were first identified as orphan members of the nuclear receptor superfamily whose ligands and functions were unknown. Two LXR proteins (α and β) are known to exist in mammals. The expression of LXRα is restricted, with the highest levels being found in the liver, and lower levels found in kidney, intestine, spleen, and adrenals (see Willy, et al., Genes Dev. 9(9):1033-45 (1995)). LXRfβ is rather ubiquitous, being found in nearly all tissues examined. Recent studies on the LXRs indicate that they are activated by certain naturally occurring, oxidized derivatives of cholesterol, including 22(R)-hydroxycholesterol, 24(S)-hydroxycholesterol and 24,25(S)-epoxycholesterol (see Lehmann, et al., J Biol. Chem. 272(6):3137-3140 (1997)). The expression pattern of LXRs and their oxysterol ligands provided the first hint that these receptors may play a role in cholesterol metabolism (see Janowski, etal., Nature 383:728-731 (1996)).
As noted above, cholesterol metabolism in mammals occurs via conversion into steroid hormones or bile acids. The role of LXRs in cholesterol homeostasis was first postulated to involve the pathway of bile acid synthesis, in which cholesterol 7α-hydroxylase (CYP7A) operates in a rate-limiting manner. Support for this proposal was provided when additional experiments found that the CYP7A promoter contained a functional LXR response element that could be activated by RXR/LXR heterodimers in an oxysterol- and retinoid-dependent manner. Confirmation of LXR function as a transcriptional control point in cholesterol metabolism was made using knockout mice, particularly those lacking the oxysterol receptor LXRα (see Peet, et al., Cell 93:693-704 (1998)).
Mice lacking the receptor LXRα (e.g., knockout or (−/−) mice) lost their ability to respond normally to increases in dietary cholesterol and were unable to tolerate any cholesterol in excess of that synthesized de novo. LXRα (−/−) mice did not induce transcription of the gene encoding CYP7A when fed diets containing additional cholesterol. This resulted in an accumulation of large amounts of cholesterol and impaired hepatic function in the livers of LXRα (−/−) mice. These results further established the role of LXRα as the essential regulatory component of cholesterol homeostasis. LXRα is also believed to be involved in fatty acid synthesis. Accordingly, regulation of LXRα (e.g., use of LXRα agonist or antagonists) could provide treatment for a variety of lipid disorders including obesity and diabetes.
In view of the importance of LXRs, and particularly LXRαs to the delicate balance of cholesterol metabolism and fatty acid biosynthesis, we describe modulators of LXRs which are useful as therapeutic agents or diagnostic agents for the treatment of disorders associated with bile acid and cholesterol metabolism, including cholesterol gallstones, atherosclerosis, lipid storage diseases, obesity, and diabetes. The agents described herein are also useful for disease states associated with serum hypercholesterolemia, such as coronary heart disease.